It has been known that stretched molded articles having a high modulus and a high tensile strength can be obtained by forming fibers, tapes or the like from ultra-high-molecular-weight polyethylene and stretching them, and numbers of patents are laid open.
For example, Japanese Patent Laid-open Publication No. 56(1981)-15408 discloses a process for preparing a stretched molded article comprising the steps of spinning filaments from a dilute solution of ultra-high-molecular-weight polyethylene and then stretching the obtained filaments, that is, so-called "gel-spinning super-stretching process".
U.S. Pat. No. 4,413,110 and U.S. Pat. No. 4,536,536 disclose a process for preparing a stretched molded article comprising the steps of preparing a dilute solution of ultra-high-molecular-weight thermoplastic crystallized polymer using a non-volatile solution, subjecting the dilute solution to spinning to form xerogel fibers, and then stretching the xerogel fibers. This process is basically identical with the above-mentioned gel-spinning super-stretching process, but in this process, there can be obtained stretched molded articles of high modulus and high strength such as those having modulus of not less than 100 GPa and tensile strength of not less than 3 GPa when an ultra-high-molecular polyethylene is used.
With respect to the ultra-high-molecular polyethylene, as described above, a process for preparing fibers having high modulus and high tensile strength has been almost established, and its theory is explained in detail in Journal of Japan Rheology Society (Vol. 13, No. 1, pp. 4-15, 1985, written by Matsuo).
Utilizing the techniques on the ultra-high-molecular polyethylene, a variety of studies have been made in order to obtain fibers having high modulus and high tensile strength from ultra-high-molecular-weight polypropylene.
For example, Kunugi et al. have obtained polypropylene fibers having modulus of 16.9 GPa and tensile strength of 0.74 GPa by stretching polypropylene having a molecular weight of 475,000 under adoption of a zone stretching process which is successful in the polyethylene art to polypropylene (Journal of Applied Polymer Science, Vol. 28, pp. 179-189, 1983). The zone stretching process means a process comprising heating in a local heating furnace a 1-2 mm portion of a fiber having been beforehand prepared by means of a conventional melt spinning method or the like, and then stretching that portion of the fiber to effect super-stretching. Further, Peguy and Manley have reported an example in which the aforementioned gel-spinning super-stretching process is applied to polypropylene (Polymer Communications, Vol. 25 pp. 39-42, 1984). In concrete, they have obtained polypropylene fibers having modulus of 36 GPa and tensile strength of 1.03 GPa by subjecting a solution having a concentration of 0.75-1.5% by weight to the gel-spinning super-stretching process in the similar process to that adopted for the ultra-high-molecular polyethylene proposed by Smith and Lemstra (Journal of Polymer Bulletin, Vol. 1, p. 733, 1979).
Moreover, the aforementioned U.S. Pat. No. 4,413,110 and U.S. Pat. No. 4,536,536 disclose a working example for preparing polypropylene in addition to the above example for preparing polyethylene, and in concrete, there is described a process for preparing ultra-high-molecular-weight polypropylene fibers having modulus of 23.9 GPa and tensile strength of 1.04 GPa using a solution of ultra-high-molecular-weight polypropylene (intrinsic viscosity [.eta.]: 18 dl/g, molecular weight: 3,300,000) having a concentration of 6% by weight.
However, when the ultra-high-molecular-weight polypropylene fibers or tapes obtained by utilizing the conventional processes for preparing ultra-high-molecular-weight polyethylene fibers are examined, the ultra-high-molecular-weight polypropylene stretched yarns or tapes obtained using any of those processes only show modulus of about 7-10 GPa and tensile strength of about 0.5-1.04 GPa.
By the way, it is known that the theoretical strength of the ultra-high-molecular-weight polyethylene is about 32 GPa, and that of the ultra-high-molecular-weight polypropylene is about 18 GPa, and the theoretical strength of the ultra-high-molecular-weight polypropylene is about 1/2 of that of the theoretical strength of the ultra-high-molecular-weight polyethylene ("Fiber and Industry", Vol. 40, pp. 407-418, 1984). At present, ultra-high-molecular-weight polyethylene fibers having tensile strength of about 6 GPa have been already obtained. For this value, the tensile strength, 0.5-1.04 GPa, of the ultra-high-molecular-weight polypropylene are not always satisfactory. That is, the tensile strength of the ultra-high-molecular-weight polypropylene should be improved to be 3 GPa, and taking the value into consideration, the tensile strength of the ultra-high-molecular-weight polypropylene now obtained is hardly improved.
An example of relatively successful processes in the improvement of the tensile strength of the ultra-high-molecular-weight polypropylene is a process reported by Kanamoto et al. (Journal of Japan Fiber Society, Drafts in the annual convention for reading research, 1987). This process comprises the steps of casting a solution of an ultra-high-molecular-weight polypropylene solution having a concentration of not more than 1% by weight and removing a solvent by means of evaporation to prepare a solvent-cast film, then subjecting the film to solid phase stretching in the pseudo melt state in such a manner that the film is sandwiched with a polyethylene buret from both sides, further stretching the film by about 6 times through a conical die, and finally subjecting thus stretched solid phase film to conventional stretching, so as to obtain highly stretched fibers having a draw ratio of about 72 times. This process uses the polyethylene buret as described above, so that a sample can be stretched in a high draw ratio without suffering any damage or break even if the sample is brittle. Concretely, in this process, an ultra-high-molecular-weight polypropylene stretched molded article having tensile strength of 2.3 GPa can be obtained using ultra-high-molecular-weight polypropylene having a molecular weight of 3,600,000.
In this process, however, the ultra-high-molecular-weight polypropylene is subjected to solid phase stretching using a conical die under the condition that the polypropylene is sandwiched with the buret, so that continuous manufacturing of fibers is difficult, resulting in disadvantages in industrial productivity and cost. Additionally, the ultra-high-molecular-weight polypropylene stretched molded article obtained by this process is extremely low in the elongation at break.
The ultra-high-molecular-weight polypropylene fibers can be generally manufactured by preparing a dilute solution of ultra-high-molecular-weight polypropylene, then spinning gel fibers from the solution and highly stretching the gel fibers.
In the case of utilizing the gel-spinning super-stretching process, however, the resulting fibers inevitably show high modulus, whereas the fibers are lowered in the elongation. Accordingly, when such fibers are intended to use as energy-regenerating elastic materials such as spring, the energy-regenerating time becomes markedly shortened because of their low elongation, so that the energy cannot be effectively stored and regenerated.
On the other hand, it has been known that fibers obtained by spinning under application of a temperature gradient and a shear stress thereto are subjected to heat treatment, so as to obtain hard elastic fibers capable of recovering elasticity without plastic deformation even after the fibers are deformed by near 100% (Fiber and Industry, Vol. 30, No. 1, pp. 18-21, 1974). As other example, it has been reported that hard elastic fibers having high elongation can be obtained by spinning fibers from polypropylene at a high speed and then subjecting the obtained fibers to heat treatment (Fiber and Industry, Vol. 36, No. 4, pp. 50-57, 1980). Furthermore, it has been reported that porous polypropylene fibers show an elongation of 40% and these fibers are suitable for energy-revival elastic materials (Japanese Patent Laid-open Publication No. 63(1988)-249711). However, the strength of these fibers are low.
In the above-described processes, heat treatment is necessarily effected in any of stages after the spinning stage, and this heat treatment is complicated, so that those processes are disadvantageous for industrially manufacturing stretched fibers having high strength and large elongation at break. In the heat treatment, moreover, the improvement in the elongation of fibers is limited to a certain level, and it is difficult to sufficiently increase an output energy value of the fiber formed from the ultra-high-molecular-weight polymer.